Every time I have done this quest the resulting fight, with fireballs and Atronach explosions galore have meant that the books are blasted randomly everywhere. Keep looking in corners etc. Though it is not impossible that they are blasted through the floor (great engine after all)
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AsdafghDec 19 '11 at 12:10

By deselecting every other active quest, the third book then had a marker over it on re-entering the Ritual room. It was almost the same colour as the floor and would have been impossible to spot without this quest marker appearing just over it.

I'm pretty sure that marker wasn't over the third book when I'd been in the room previously (when I had multiple active quests).

The quest highlights the books with a giant arrow. I found two on a pedestal, and the third one knocked over by the fighting. Since they get thrown around so easily by all the magic, it is possible it was knocked through a wall.

I can't find any of the books. I'm in the Fellglow Keep? Are they in the dungeon further down?
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user14138Nov 13 '11 at 5:45

they are in the ceremonial room where the final battle is.
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Patrick KlugNov 13 '11 at 7:42

there was definitely no arrow pointing to the third book in my case.
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Patrick KlugNov 13 '11 at 11:16

1

Whether it's possible to grab the book before the fighting starts or not might depend on your dialog choices. Considering to leave the prisoner there gives you fee-roam without the Caller being aggro until you touch the first book...
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KonerakNov 14 '11 at 9:46

I had the same problem, but then had an auto save file save over the battle that took place. If this happens to you, and you can't restart from just before the battle, try pressing start, opening up your mission menu and pressing X over "Find three books". In my case, the arrow hovered directly over where the book was in the room, which was knocked over by the fighting.